I watched the animated film Storks the other day. There are many silly things about this film, but the one that stuck in my throat was this.

Here is a stork.

Here are the storks in Storks.

Those are seagulls. Look at the heads. Look at the beaks. Seagulls.

This was bugging me the next morning such that I was forced to tweet.

There’s an obvious answer to that which Chesterton sums up very well in one of the Father Brown stories:

“It really is more natural to believe a preternatural story, that deals with things we don’t understand, than a natural story that contradicts things we do understand. Tell me that the great Mr Gladstone, in his last hours, was haunted by the ghost of Parnell, and I will be agnostic about it. But tell me that Mr Gladstone, when first presented to Queen Victoria, wore his hat in her drawing-room and slapped her on the back and offered her a cigar, and I am not agnostic at all. That is not impossible; it’s only incredible. But I’m much more certain it didn’t happen than that Parnell’s ghost didn’t appear; because it violates the laws of the world I do understand.”

AKA: they’re bloody seagulls. Obviously.

Chesterton’s explanation is true as far as it goes: if a book presents us with something that we know to be wrong, without explanation, we don’t accept it. Obviously, if London is made of sentient jelly which has the power to suck down Tube stations and spit them out again in different places and that’s why Oxford Circus is now south of the river, that’s a perfectly good reason. I will happily suspend my disbelief, if you just give me a hook to hang it off.

But I think there’s more to dig out here for worldbuilding purposes, and it was brilliantly put by Twitter user @aprotim.

For an implausible thing to feel right and true in a story, it must have a reason. If there isn’t a reason, it’s unconvincing. But if every implausibility has a different reason, what you get is a mess.

In the alt history programme SS-GB we accept any amount of divergence from reality because it all flows from the same point of deviation: the Nazis won. (And therefore Churchill is dead, and therefore swastikas everywhere, etc.) We accept all that immediately from the basic premise. However, if SS-GB decreed that everyone in the UK was legally obliged to have a cat, we’d all be sitting up and saying, “What?” because that doesn’t arise from the premise. It requires us to be given and accept a second, unrelated explanation. (“In this reality Hitler was super fond of cats.”) It’s not just that it deviates from the real world in which I live; it also diverges from what I thought to be the case for the fictional world in which the Nazis won.

And this is the point about economy of deviation. Deviations that come back to a single premise (“there are ghosts”; “the city is made of jelly”; “people have superpowers”) can be the root of a massive branching and flowering tree of story, and lead to all kinds of weird and wonderful things, and we’ll happily go with them because they flow from the initial premise. But unrelated deviations requiring separate explanations—or, worse, which are unexplained–sap at the verisimilitude of the story because we like things to fit.

There’s a famous statistics puzzle that goes as follows:

Linda is 31 years old, single, outspoken, and very bright. She majored in philosophy. As a student, she was deeply concerned with issues of discrimination and social justice, and also participated in anti-nuclear demonstrations.

Which is more probable?

1.  Linda is a bank teller.
2.  Linda is a bank teller and is active in the feminist movement.

The correct answer is 1. It is more probable that one thing will happens than that two will. (Think of it this way: the odds of A happening are better than the odds of A happening and also B happening.)

But this is counter intuitive for humans. The majority of people will go for option 2, and this is why we say lies, damn lies and statistics. We have been given a story that leads us to feminist and not to bank teller, therefore bank teller alone is a less plausible outcome for humans than feminist bank teller because it doesn’t fit the story. It diverges from the facts we have; it requires a second explanation; it isn’t convincing. Option 1 may work for statisticians; it doesn’t work for novelists at all. (This is the principle of Occam’s Razor and Chekhov’s Gun: we don’t want dozens of different reasons for things.)

To return to Storks: I am not bothered by the base concept of “storks actually create and deliver babies” because that’s a given for the universe. I am also not bothered by the stork having teeth inside its beak

horrifying though that is, because anthropormorphism is part of the animated universe. Those are both givens of the story. But I am bothered by the storks looking like seagulls, because that is a divergence from my world which is unexplained by anything in the film. It’s not based on anything; it doesn’t lead from or flow to anything. It was done for the convenience of the animators (just as a pivotal row in a romance novel may arise because the author feels “we need a row here” rather than out of the characters and their situation). And as such, it feels troubling, annoying, and deeply implausible in a film which features a submarine made of wolves.